The present invention relates to ionization detector devices. More specifically, the present invention relates to an apparatus and a method of making a glow discharge electron capture detector having a small detection volume.
Ionization detectors are well known in the art. These detectors are typically used in conjunction with a gas chromatography system (xe2x80x9cGCxe2x80x9d) in order to detect the presence of specific compounds. Sample gas flowing from the GC is typically received by an ionization detector where it undergoes an ionization process. In this process, molecular compounds within the sample gas are ionized and the ionization of these compounds is detected and measured by an electrode within the ionization detector.
One such ionization detector is called an Electron capture detector (xe2x80x9cECDxe2x80x9d). An ECD is a type of ionization detector which offers high sensitivity and high selectivity towards electrophilic compounds and is widely used for detecting trace amounts of pesticides in biological systems and in food products. Such compounds typically contain halogens which readily combine with free electrons that are created within the ECD. The resulting decrease in free electrons in the ECD is used by the ECD as an indication of the concentration of the compounds in a sample.
In more traditional methods, the creation of free electrons within the ECD is typically achieved through the use of a radioactive source, such as radioactive Nickel 63. A detector gas capable of producing thermal electrons such as nitrogen flows through a chamber containing the radioactive source. The radioactive radiation causes ionizing particles, in the form of positive ions and free low energy electrons, to be produced. These free electrons are readily accepted by electrophilic molecules within the sample gas, otherwise referred to as analytes.
The chamber also typically includes an electrode which detects a current flowing within the ECD produced by the flow of free electrons. The level of this current provides an indication of the concentration of analytes within the sample gas.
While prior ECD work for their intended purpose, these devices have a number of drawbacks that need improvement and modification. One such drawback is the use of radioactive source for ionization. The use of a radioactive source requires specialized procedures for handling and use. These additional procedures add significant cost and time to operate the GC. Furthermore, the use of a radioactive source within the ECD poses a potential health risk to the operators of the GC.
There are other types of ionization detectors which do not use the radioactive materials employed by typical ECD. However, these non-radioactive ionization detectors typically do not have a large linear dynamic range of like that typically found with state of the art ECD.
Current ECDs, as well as most ionization detectors, also typically require relatively large detector volumes of approximately 150 xcexcL or greater. As a result of the relatively large detector volumes required, large amounts of sample gas and high gas flow rates are also required to properly operate the ECD. This requirement can be problematic for a GC, especially for ultrafast and portable gas chromatographs.
Furthermore most ionization detector designs are also plagued by structural breakdowns which affect the ECD""s accuracy. Ambient air leaks often develop within the detector seals. Ions often collect on the signal electrode surface, as a result of its direct exposure to sample gas flows. These type of structural breakdowns can significantly affect the accuracy and reliability of an ECD.
Therefore a need exists for an apparatus and method which overcomes the aforementioned problems currently known in the art.
The present invention provides an advantage in ionization detector design by providing an ECD that does not utilize radioactive material as a source for ionization particles. Such an ECD reduces the time and expense attributed towards the handling requirements of the radioactive material. Furthermore, the use of non-radioactive materials eliminates a potential health risk for the operator of the GC.
The present invention also provides an ECD having a smaller detector volume relative to those ECDs known in the art. This enables the present ECD to achieve greater accuracy without requiring the presence of larger amounts of analytes and high gas flow rates.
The present invention also provides an ionization detector which minimizes the effect of ambient air leaks on the detector""s accuracy.
Accordingly, the present invention avoids the drawbacks of prior ionization detectors and provides additional benefits as a result of its design. This improvement over the prior art is achieved through the use of a unique combination of elements in its design.
In one embodiment, the ionization detector includes a unique combination of elements including a non-radioactive glow discharge source, a gas flow area designed to promote mixing of sample gas with a reagent gas, a GC column interface which directs air leaks away from the sample area, and a signal electrode designed to discriminate against positive ion collection.
One embodiment includes a body defining a detector cavity and a sample flow area. The detector cavity is essentially funnel-shaped and extends longitudinally through the body. The sample flow area includes a frusto-conical passage in communication with the detector cavity.
One embodiment also includes a plurality of electrodes disposed within the body. The electrodes include a glow discharge source comprising a first and second discharge electrode located near one end of the body, a guard electrode located longitudinally intermediate to the discharge electrode and the sample flow area, and a signal electrode disposed within a signal electrode cavity located near an end opposite to the discharge electrode.
A column interface depends from the body and is in communication with the sample flow area. The column interface includes a reagent inlet and a purge outlet which extend laterally from a longitudinal portion of the column interface. A column from a GC extends through the longitudinal portion and is partially disposed within the sample flow area.
While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, wherein is shown and described only the embodiments of the invention, by way of illustration, of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.